I've head some people say you need to run a diesel hard, otherwise you may shorten it's life.

I've heard some people say, they only cruise at a very moderate rpm, and that constantly running at a high rpm is bad for it.

And I've recently heard someone say that the first one is only true with old diesels.

So which one is actually true?

I just spent a whole lotta money on a new diesel and I listened very carefully to what this highly respected diesel mechanic had to say.

I was told to break the engine in for 50 hours. And by breaking it in, he meant idling it as little as possible, and running it near its upper limits as much as possible. He said that way the moving parts would wear in a pattern that would help the engine when I needed that burst of speed, say going under a bridge against a current.

He said once the engine was broken in I should run it at the higher end whenever possible, and that running a diesel at medium and especially low RPM's shortens their lives.

I have no first hand knowledge of this because I don't know how my old engine was run before I bought the boat.

This opinion is worth exactly what you paid for it, because you will never know the mechanic in question and be able to evaluate the wisdom of his advice. But I trust him 110%.

I base my engine speed on economy vs speed. I seldom run full out because it uses too much fuel. I don't run it real low because I believe it needs more load although a three blade prop and a high output alternator helps to provide some load. So on my Westerbeke 46 I usually cruise at around 2200 rpm which gives me 6 kt or so and keeps fuelconsumption modest. If I crank it up any more I only gain about 1 kt and burn fuel like there was no tomorrow.

I have some relatively new Cummins and the owners manual along with the Cummins people themselves are saying the same thing as Raku.

Diesels will not last as long if they are run at low RPM's most of the time or are lugged by having too large of a propeller. They also need to have the carbon burned out of their combustion chambers periodically by running them up to a full load or very close to a full load.

What Cabo says is also true. To double your speed you must cube your fuel consumption. So what it comes down to is greater fuel economy by running them slow or greater engine longevity by running them fast. So pick your poison, there is no perfect solution.

SOME YANMAR DIESEL ENGINE TIPS:
Sizing:Rule of thumb for Horsepower Required:
Sailboat or displacement launch: one horsepower per 500 pounds ( plus/minus 50lbs ), one kilowatt per 170kg of fully loaded boat is enough to punch into an average seaway.

Operating Temperatures & RPM:

Fresh-Water Cooled engines do not require long warm up periods. By the time you start the engine, check all clear, cast off and get out of the marina or in a clear area the temp will be over 60 Deg. C (140 F), displacement vessels can go straight to cruising power, once clear of the marina.

Prolonged no-load operation of the engine will cause it to blue/gray smoke at low rpm with an associated increase in oil consumption.For every 3-5 hours low load operation (trolling) or no load operation (running the fridge or charging batteries) should be countered by 20 minutes near maximum continuous rpm.

Raw water cooled engines: As the operating temp is only 55 deg. C (131F), provided the engine will keep running in gear, the clutch can be engaged. Apply cruising power about 5 minutes after you start moving.

When increasing engine rpm take about 10 seconds to move the throttle lever between low load and full load.

Because a Raw Water (salt) Cooled Engine operates at about 55 Deg. C (130 F), you can keep your hand on the cylinder block while the engine is running. If the temp goes over this the salt will precipitate out of the water and restrict the water galleries at an unacceptably fast rate. Also, because of this, the alarm operates at 62 Deg. C (144 F). The raw water cooled engine thermostat starts opening between 104 -112deg F and is fully open at 125 deg F. Because the reading is quite low, most gauges are often inaccurate, except test gauges (not usually practical and very expensive).

Even with freshwater cooled engines, that operate at higher temperatures, the oil retains the byproducts of combustion and always looks black.

The best way to help get rid of oil contaminants is to rev up the engine for a half minute (over 3000 rpm), out of gear, before moving the lever back to idle and shutting down. Do this at the last stop of the day if stopping and starting a lot. Also run the engine at 3000 rpm or over for 10 minutes or more, for every 5 hours of slower running. 2000- 2500 rpm is very slow. The engine will prefer 2800 rpm or thereabouts.

Do not operate for extended periods below 80%maximum rpm. The cylinder bores will glaze, causing an increase in oil consumption and blue smoke. Don't 'baby' your diesel engine!
TIP:
If you’ve ever forgotten to turn on the raw water supply, before starting the engine - put the engine key on the sea cock every time you turn it off.

For engines without a tachometer, run it where it sings best. If the ship is shaking and growling, alter the RPM. If the engine VIBRATES unpleasantly at a particular rpm, don't operate it in that rpm band or it will eventually break!

Yanmar TACHOMETERS usually read about 100rpm too high at maximum rpm (and they aren't adjustable), so don't be pedantic about lining up the needle exactly.
You may note that the MAXIMUM RPM of your engine under load is higher than the figure I have given below.

USE THE MAXIMUM RPM IF YOU ARE IN TROUBLE, don't be frightened, the engine will handle this for an hour, provided it has been installed correctly. Once out of trouble, reduce rpm to 85%, for better fuel consumption and longer engine life.

For prolonged running, a pleasure boat engine should operate at about 85% of maximum continuous RPM.

CRUISING RPM (Yanmar):
GM, JH, JH2 series about 2900rpm

Propellers:

The propeller controls the maximum rpm under load. On Sailboats, there is no substitute for propeller diameter. Fit the largest diameter prop that will fit in the space (aperture) available*. You need to take into account the available gearbox ratios.
e.g. a 3GM30 with a 2.36:1 ratio gearbox will swing a 15 inch (380mm), a 2.61:1 ratio will swing a 16 inch (405mm) and a 3.2:1 ratio will swing an 18 inch (460mm) diameter propeller.

*Prop’ Clearances Rules of Thumb:
- The clearance between the tip of the prop and the hull = about 15% of Propeller Diameter
- The clearance between the rudder and the nearest point of the propeller = about 15% of Prop’ Diameter

- The clearance between a full keel and the leading edge of the propeller, measured half way out along the blade from the center of the shaft = about 30% of Prop’ Diameter, at half the radius from shaft centreline.
- The clearance between the prop strut bearing and the propeller hub = about equal to or 1.5 times Shaft Diameter.

Fit as large a diameter propeller as possible, taking into account the available gearbox ratios.
e.g. a 3GM30 with a 2.36:1 ratio gearbox will swing a 15 inch (380mm), a 2.61:1 ratio will swing a 16 inch (405mm) and a 3.2:1 ratio will swing an 18 inch (460mm) diameter propeller.

Propeller pitch is determined by boat speed and is calculated for individual boats.

Caution All engines: If the engine does not start on the first attempt, turn off the raw water cock. When the engine starts, turn on the cock immediately. Prolonged cranking with the raw water turned on will fill the engine cylinders with water!

Bleeding GM Series Engines:

1. Open the small BLEED SCREW on top of the engine mounted fuel filter and operate the fuel liftpump by hand.

2. After the fuel filter has been purged of air, close the bleed screw (don't do it too tight, it's only little) and open the one on the fuel injection pump.

3. Again, after the air has been purged, close the bleed screw.

- The engine should now start, if not you will have to bleed the high pressure side as follows:

4. Crack the pipe nut at each injector, open the decompression levers and rotate the engine with the starter motor.

5. When the air appears to have been purged (the fuel looks clear, not whitish or frothy), tighten the nuts firmly, close the decompression levers and start the engine.

1. If the HAND PRIMING LEVER doesn't have any resistance through any of its travel, rotate the engine crankshaft through 360deg. Use the starter motor or crank handle.
There is no resistance to the hand priming lever because the internal actuating arm is on the top of the cam that drives it. Rotating the engine crankshaft 360deg will turn the camshaft 180deg, the arm will now be on the back of it's cam and you should feel resistance when operating the hand priming lever.

2. Put a rag or paper towel around a loosened bleed screw or injector pipe nut to retain the diesel that will leak out.

3. Use a folded paper towel or toilet paper to check around joints for leaks. Leaking fuel will quickly be absorbed by the paper and stand out like the proverbial....
4. Clean the engine bay, or any other affected area, with a lemon scented dishwashing liquid to get rid of any diesel smell.

5. One lousy day, when you're bored and have nothing else to do, paint the heads of the bleed screws red, it will help you find them when the heart is pounding, the stomach is heaving and the S#*T is about to hit the fan.

6. SB, YSB and YSE engines: To bleed the injector line the throttle lever must be in the full power position.

7. Some engines have a HAND PUMP on top of the fuel filter, push it up and down. You may not need to open any bleed screws as the aerated fuel is sent back through the return line to the tank.

WINTERIZING or De-Commissioning:

RAW WATER COOLED ENGINES:

1. Open the cylinder block, exhaust mainfold and waterlock drains, leave them open.

2. Drain the water intake filter and leave the seacock open.

3. Remove the air cleaner, open the decompression levers. While turning the engine over with the starter motor, spray CRC 'Longlife' into the air intake for about 5 seconds per cylinder.

4. Close the decompression levers and refit the air cleaner. Put a plastic bag or similar over the aircleaner so air can't get in.

5. If you have an exhaust outlet valve at the transom, close it and put the engine key on it !

5b.If you dont have a valve, put a rag up the exhaust and seal it with plastic and tape or something so that air can't get in. Do something clever with the engine key so that you remember to remove the obstruction before starting the engine !

6. Wipe a thin smear of grease over the control cable end where it attaches to the engine. Do the same for the stop cable.

7. Remove the battery and take it home to charge it occasionally. If you can't remove the battery at least remove the terminals and give them a light grease, leave them off till Spring.

1.If the coolant needs replacing, do it now. N.B. You need to be able to run the engine to do this properly.

2. Open the raw water drains on the heat exchanger manifold, gearbox oil cooler and where ever else they are on your engine. Open the waterlock drain.

3. Remove the silencer. While holding the stop button, press the starter button and spray CRC 'Longlife' into the air intake. Take about 10 seconds per cylinder with stops every 10 seconds to admire the view. ( and let the starter cool down )

Caveat emptor: E. & O. E. - While every endeavour is made to ensure the information in this posting is correct, please email me if you find any mistakes so that they can be corrected.

Yanmar Break-In Procedure:

Prolonged idling of diesel engines is not good for them.

For the first 30 minutes, maintain idle to about 1500 RPM, until the engine temperature has stabilized, then advance to Maximum Loaded RPM, in 200 RPM increments, over about 15 -20 minutes.

If the engine can't reach its correct maximum RPM then slow down, return to the dock, and have it fixed or explained.

For the next 5 hours running don't operate the engine at more than 85% RPM (nor less than 2000 RPM) and every 20 minutes change the RPM up or down by 200 RPM. When the 5 hours are up, you’re good to go cruising.

__________________
Gord May "If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

There are many "Types" of diesels out there. Older, heavy slow turning to newer lightweight hi rpm types. The lighter one like the Yanmars, Kobota and Mitsubishi based units under 100 hp do not do well at low rpm with hi load. They really need to get rpm to make HP. Lugging them will cause issues like carbon in the rings and compression loss. My rule of thumb your cruising speed should be about 70 -75 % of Max RPM. My max RPM is 3600 where it is prop to but I cruise at about 2500-2600. I think it is a good practice to run the throttles up to max rpm for a few minutes every 10 or 20 hours.

I have worked on large diesels that max out at 200 - 300 rpm and are the size of a bus. Things are much different with the new small displacement low mass engines we see in boats these days.

I have some relatively new Cummins and the owners manual along with the Cummins people themselves are saying the same thing as Raku.

Diesels will not last as long if they are run at low RPM's most of the time or are lugged by having too large of a propeller. They also need to have the carbon burned out of their combustion chambers periodically by running them up to a full load or very close to a full load.

What Cabo says is also true. To double your speed you must cube your fuel consumption. So what it comes down to is greater fuel economy by running them slow or greater engine longevity by running them fast. So pick your poison, there is no perfect solution.

So true with almost everything regarding boats, especially sailboats -- everything's a compromise.

Currently have about 10,000 hours each, and 95% of that time has been at a power level of 10-20%, based on fuel flow rate. No special effort to rev to max occasionally. Lots of "trawler miles" at 6 kts.

Oil consumption is nominal, no smoking, rev happily to max when I need it.

So I conclude that my engines are happy enough at low power levels, and have no plans to change my behavior.

I think a key point is that these engines have so-called "water jacket oil coolers", which in this case actually warm the oil to coolant temp (195F) continuously, and so reduce sludging due to condensation. Just a theory....

My understanding is that the big thing is to operate the engine at proper operating temp. This means oil temp. as well as water temp. Some engines have an oil "cooler" cooled by engine coolant, not sea water. This is really how they regulate the oil temp. and bring it up to proper. operating temp. and keep it there regardless of engine speed.

Next, proper and timely MAINTENANCE. I think many boat owners forget about the actions they need to take to keep their engines healthy.

Many trawler engines have run for years at very low power level and remain healthy.

I have two Cummins BT5.9M, 220 hp, in a 50' trawler catamaran. 11 years old.

Currently have about 10,000 hours each, and 95% of that time has been at a power level of 10-20%, based on fuel flow rate. No special effort to rev to max occasionally. Lots of "trawler miles" at 6 kts.

Oil consumption is nominal, no smoking, rev happily to max when I need it.

So I conclude that my engines are happy enough at low power levels, and have no plans to change my behavior.
YMMV

Cheers
Dave

Have the same engine in my Dodge pickup. 6,000+/- hours (280,000 miles) 80% of the time at a power level of 18-20%, based on fuel consumption.

Have a Cummins 6CT 8.3, 300hp in my charter boat with 3,000 hour since rebuild (bought it used). It runs 75% of the time at a power level of 80% based on fuel consumption.

Both engines run happily and neither burns any oil. Both get plenty of clean air, clean dry fuel, and regular oil changes. So beleive what you want about how to run a diesel engine.

It should be noted that both engines have the lowest horsepower rating (with turbo) for their block size. Both engines sizes are available with 50%+ greater horsepower ratings. I would guess Yanmars are much closer to maximum horsepower for the particular block size.